Which of these numbers is prime? ${9,\ 19,\ 33,\ 39,\ 80}$
A prime number has exactly two factors: 1 and itself. The factors of 9 are 1, 3, and 9. The factors of 19 are 1 and 19. The factors of 33 are 1, 3, 11, and 33. The factors of 39 are 1, 3, 13, and 39. The factors of 80 are 1, 2, 4, 5, 8, 10, 16, 20, 40, and 80. Thus, 19 is a prime number.